1. Field of the Invention
The present invention relates generally to port power switches, and more specifically to methods of lead compensation that use port power switches.
2. Description of the Related Art
The Universal Serial Bus (USB) was developed to offer PC users an enhanced and easy-to-use interface for connecting an incredibly diverse range of peripherals to their computers. The development of the USB was initially driven by considerations for laptop computers, which greatly benefit from a small profile peripheral connector. Among the many benefits of the USB is a reduction in the proliferation of cables that can affect even the smallest computer installations. In general, USB has become the interface of choice for PCs because it offers users simple connectivity. USB eliminates the need to have different connectors for printers, keyboards, mice, and other peripherals, and supports a wide variety of data types, from slow mouse inputs to digitized audio and compressed video. In addition, USB devices are hot pluggable, i.e. they can be connected to or disconnected from a PC without requiring the PC to be powered off
The USB specification has seen various revisions, with the USB 2.0 standard challenging the IEEE 1394 interface (“Firewire”) as the interface of choice for high-speed digital video, among others. The USB 3.0 standard, representing the second major revision of the USB standard, specifies a maximum transmission speed of up to 5 Gbits/s (640 MBbytes/s), which is over 10 times faster than the maximum speed specified in the USB 2.0 standard (480 Mbits/s). The USB 3.0 standard also features reduced time required for data transmission, reduced power consumption, and is backward compatible with USB 2.0. A connection between the USB device and the host may be established via a four-wire interface that includes a power line, a ground line, and a pair of data lines D+ and D−.
The USB standard provides guidelines for the allowed common-mode voltage on the differential data lines (D+ and D−). Newer specifications also allow for battery charging using a USB port, which is oftentimes implemented through port power switches (PPS) incorporated in a USB host and/or hub. More commonly, traditional ‘linear-type’ PPSs utilized in USB and other DC power applications, e.g. in PCs and notebook computers, serve to provide or prevent power application to one or more attached electronic loads. These low cost protection devices are a commodity and have proliferated in the market.
USB ports typically each include one PPS, which serves to protect both the application (e.g. in a USB device) and electronic load from certain types of failure, such as electronic load short circuit or an application over-voltage. According to USB-IF specifications, a USB port is required to provide between 4.75V and 5.25V for non-dedicated charging ports configurations. Examples of electronic loads include USB portable devices such as cell phones attached via the applications USB connector, e.g. the Point of Load (POL). A typical primary DC power source in these applications is a Switch-Mode Power Supply (SMPS) that provides high efficiency voltage conversion from the internal higher voltage battery voltage to a lower voltage, such as 5V DC +/−5%, for both the internal system and one or more attached electronic loads.
Some electronic loads base their rate of charge on the voltage level present. For example, if 5.25V is present, charging could be at 2.0 amps. However, if the voltage present is 4.75V, charging current could drop down to 1.0 amp. This results in doubling the charging time and is undesirable. Since linear PPS devices contain a finite amount of ‘ON’ resistance during operation, increasing the electronic load current will cause a corresponding increase in voltage drop across it (according to Ohm's law). Furthermore, circuit board resistance can further increase this voltage drop. Portable devices with larger batteries require more charging current in order to charge within a reasonable amount of time, thus requiring the SMPS voltage output to be set to a higher voltage level to compensate for any expected application voltage drops under load. Unfortunately, when no load is present, this voltage could exceed the USB-IF limit of 5.25V.
Since SMPS applications depend on voltage feedback in order to maintain their voltage output under varying load conditions, the ideal point to monitor is the POL. However, this is problematic when the PPS enters a fault condition and shuts ‘OFF’ as the POL voltage reaches 0V in most cases. This causes the SMPS to attempt to increase its voltage and enter a fault state. Accordingly, SMPS feedback reference points are either at their voltage output pin or at the input to the PPS switch, which does provide some compensation for the printed circuit board (PCB) resistance from the SMPS to the PPS. However, the PPS “ON’ resistance and trace resistance from the PPS voltage output to the POL is not compensated.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.